The reduction of harmful emissions from engines (or other systems converting thermal energy to mechanical energy) plays an important role in efforts to meet ever more demanding environmental protection requirements. Apart from emissions from the combustion process itself, this also concerns emissions produced by subsidiary processes in or on the engine. They may be not only emissions that can be regarded as reactants of the combustion process but also emissions that result from amounts of oil entering the exhaust gas of the engine, whether due to being scraped off from walls or entering in the form of vapors or droplets. In order to be able to achieve a reduction in the emissions, it is required to detect and assess the nature and type of emissions. For this purpose, the oil emissions in particular of uncombusted hydrocarbons must be measured, and this must be carried out at high speed over a great mass range in order to be able to obtain an impression even of processes occurring internally in the engine with sufficient dynamics.
This involves in particular the determination of oil emissions that are caused by various mechanisms. On the one hand there is evaporation, which in particular occurs with greater probability for higher-volatility molecules than for low-volatility molecules, depending in turn on the thermal energy. Another important mechanism is a parasitic flow in the combustion chamber (reverse blow-by), which represents a compensating gas flow and can be encountered in the reciprocating piston engine, in particular in the region of the piston rings and the piston grooves. It leads into the combustion chamber and thereby carries oil with it in the form of droplets. Finally, there is another important mechanism, that of oil being scraped off and/or thrown off by mechanical forces, so that the oil is torn out in the form of droplets of the oil and gets into the combustion chamber or the exhaust-gas flow.
Various measuring principles are known from the relevant prior art. A first measuring principle is based on chemiluminescence or UV fluorescence for the analysis of oil combustion residues and/or tracer substances. In the case of this measuring principle, the oil consumption can only be dependably measured if the oil constituents are completely converted into combustion residues, for example in the form of sulphur dioxide SO2. In principle, depending on the mixture forming parameters, a sufficient amount of oxygen or thermal energy is not available reliably for this in the combustion exhaust gas. Consequently, an oxidation furnace is therefore additionally provided to ensure complete combustion. An operating pressure which must not deviate too much from the ambient pressure is required for the combustion. In order to ensure a frictionless sample gas transfer, this operating pressure may only have a small pressure difference from the location from which the gas is taken. This may lead to restrictions with respect to the dynamics. Furthermore, the furnace itself represents a considerable low-pass in the measuring chain that restricts the applicability for dynamic measuring tasks. A further major disadvantage of this measuring principle is that detectors for UV fluorescence in particular have a pronounced cross-sensitivity with respect to other combustion residues or exhaust-gas constituents, so that signals that are not ascribable to the actual lubricating oil cause a falsification of the measuring results.
There is a further known measuring principle, in which the measuring gas is analyzed by mass spectrometry for oil combustion residues and/or tracer substances. This has the advantage over the aforementioned principle of lower cross-sensitivity with respect to further combustion residues or other exhaust-gas constituents. However, it must similarly be operated with a furnace for post-combustion. This gives rise to the same disadvantages with respect to the dynamics as in the case of the previously described method.
A further measuring principle is based on radioactivity. This involves collecting a radioactive element previously incorporated in the hydrocarbon chains of the lubricating oil by means of a filter or a condensate trap, and finally measuring it by means of a radioactivity detector. The handling of radioactive sources requires particular care and is therefore laborious. Moreover, the measuring result is influenced by the filtering characteristics or the capability of the condensate trap to form a corresponding condensate. Furthermore, it has been found that, if there is a lack of thermal energy, oil droplets such as occur in particular in the coasting mode of the engine do not reach the collection point, but are already deposited beforehand on walls of the line. Consequently, a falsification of the measuring result in the direction of lower values occurs.
A further measuring principle is known from DE 10 2004 001 514, in which uncombusted constituents of the lubricating oil are fed to a high-pass mass filter configured as an electrical multipole and are subsequently subjected to mass spectrometry. The measuring device itself has high dynamics, and to this extent meets requirements. However, its performance with respect to the detection of oil emissions in the form of droplets is unsatisfactory.